Bug 1035599: Merge the mozilla-central and comm-central objdir, r=Standard8, a=me for CLOSED TREE

This change merges mozilla-central and comm-central into having a single
topobjdir file but retaining two topsrcdirs (and two build systems). This state
is hopefully only the first part of a series of changes that eliminate the
comm-central build system partial clone completely.

In other words:
$(DEPTH) has accessible, addon-sdk, ..., mail, mailnews
$(topsrcdir) has mail, mailnews, ..., mozilla

--HG--
rename : bridge/bridge.mozbuild => mailnews/mailnews.mozbuild